chore: add CLAUDE.md and Claude Code skills for plugin development

- CLAUDE.md: project guide covering structure, plugin API, testing framework,
  build commands, and CI pipeline
- .claude/skills/new-plugin: scaffolds a complete new instrumentation plugin
  (code, tests, docker-compose, expected data, services)
- .claude/skills/plugin-test: runs plugin/unit tests locally across Python
  versions, mirroring CI pipeline

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wu-sheng